Subscription propagation in a high performance highly available content-based publish/subscribe system

ABSTRACT

The present invention is directed to a publish/subscribe system containing a plurality of brokers, a plurality of subscribers and plurality of brokers including publisher connecting brokers, intermediate brokers and subscriber connecting brokers. Subscriptions are introduced into the system by the subscribers through associated subscription brokers. New subscriptions are aggregated, assigned a virtual start time and propagated through the system toward the publishers. Each broker maintains subscription information in the form of a directed acyclic graph and a broker vector. Messages are published through the system by the publishers through their associated publisher connecting brokers. Each message is assigned a message vector associating subscriptions to that message. The published messages are routed through the brokers toward the subscribers in accordance with comparisons of message brokers and vector brokers conducted at each broker.

FIELD OF THE INVENTION

The present invention relates generally to content-basedpublish/subscribe systems and more particularly to methods forpropagating subscriptions and routing published messages from publishersto subscribers. Therefore, the invention can be utilized in systems thatsupport reliable (i.e. in-order, gapless) delivery of messages utilizedin addition to best-effort delivery.

BACKGROUND OF THE INVENTION

In content-based publish/subscribe systems, publishers post or publishinformation through the system, and subscribers specify interest inreceiving certain information. Subscriber interest is specified usingsubscriptions that define predicates on the posted information. Toimprove performance in the delivery of information in thepublish/subscribe system, broadcasting every message throughout theentire publish/subscribe system is avoided. In a content-basedpublish/subscribe system, the information requested varies bysubscriber, and any one subscriber may only be interested in a verysmall portion of the overall amount of information published. Forexample, when the subscriptions in the publish/subscribe system overlapsignificantly and match only a small subset of all of the publishedinformation, most of the published information does not need to bebroadcast throughout significant portions of the publish/subscribesystem.

Publish/subscribe systems typically prevent this unnecessary flooding ofpublished information throughout the system by propagating thesubscriptions through the system to the publishers of the informationand by using the propagated subscriptions to direct the routing ofinformation through the system and to filter out published informationthat does not have to be routed. This routing and filtering isfacilitated by the use of brokers disposed between the publishers andsubscribers in the publish/subscribe system. Each broker contains thepropagated subscriptions that are relevant to the subscribers to whichthe broker can route published information. In particular, each brokercontains subscriptions for neighboring brokers and subscribers in thesystem. The broker uses the propagated subscriptions to filter thepublished information in accordance with the subscriptions as theinformation passes through the publish/subscribe system. This process isreferred to as message filtering and can be used anywhere in thenetwork, even in brokers that are in close proximity to the publishersthat are posting the information. Therefore, only information that isrelevant to downstream subscribers connected to the brokers isforwarded, and published information is not forwarded to brokers andsubscribers in the publish/subscribe system that are not associated withmatching subscriptions.

As more subscriptions are added to and propagated through thepublish/subscribe system or as proximity to the publishers increases,the amount of information about subscriptions that is maintained in eachbroker for the purpose of information filtering and routing grows andcan become cumbersome, adversely affecting the performance of thepublish/subscribe system. Conventionally, publish/subscribe systemsattempt to avoid this problem by using subscription aggregation orsubscription consolidation. In subscription aggregation, for example, ifinformation that matches a first subscription also matches a secondsubscription and both subscriptions are routed in the same directionwith respect to a given broker, then only the second subscription ispropagated. This aggregation process utilizes covering relationshipsamong the subscriptions, and many publish/subscribe systems thatimplement subscription propagation utilize covering relationships amongthe subscriptions to reduce the volume of information propagatedthroughout the system and maintained at each broker.

In addition to attempting to match published information withsubscriptions as efficiently as possible, publish/subscribe systems areoperated to provide in-order, gapless delivery of published information.The need for providing in-order, gapless delivery of information, evenin the presence of system failures, arises from service level agreementsthat dictate the need for an uninterrupted flow of information, e.g., itis unacceptable for certain stock traders not to be able to access atrade event that others can access, and from message interdependencies,for example when messages are used by a subscribing application toaccumulate a view of an event and missing or re-ordered messages cancause an incorrect state to be displayed. Achieving the requirementswithin a single system for in-order, gapless delivery, high performance,scalability and high availability using conventional methods is verydifficult.

Loss of connectivity by subscribers, publishers and brokers is common inwide-area network applications due to hardware and software failures andnetwork mis-configurations. To increase system availability, somepublish/subscribe systems are built on a redundant overlay network,which provides redundancy in the underlying network links. However,current systems do not efficiently exploit the available redundancy inthe overlay network to recover from hardware and software failures in atimely and efficient process. In a typical redundant overlay network ofbrokers, multiple paths may exist between any two brokers in thenetwork, and the publish/subscribe system automatically load balancespublished information traffic across these paths. When one of thesepaths is broken, for example due to a broker or link failure, thepublish/subscribe system redirects the published information traffic toavailable alternate paths.

Conventional methods used to provide reliable delivery in redundantoverlay networks, however, store persistently any messages or messagemeta-data on the routing path between publishers and subscribers. Anapproach to supporting reliable delivery without persistently storingmessages or message meta-data on the routing path is described in(reference). However, that approach does not consider dynamicsubscription changes caused by subscribers connecting or disconnectingfrom the system.

Known publish/subscribe systems that can handle dynamic subscriptionchanges do not provide gapless, in-order delivery and do not utilizeredundant paths existing in the broker networks. Therefore, the knownsystem are not highly scalable and available.

Examples of publish/subscribe systems that support subscriptionaggregation to achieve scalability are found in A. Carzaniga, D. S.Rosenblum, and A. L. Wolf, Design and Evaluation of a Wide-Area EventNotification Service, ACM Transactions on Computer Systems,19(3):332-383, August 2001 and R. Chand and P. A. Felber, A ScalableProtocol for Content-Based Routing in Overlay Networks, Proceedings ofthe IEEE International Symposium on Network Computing and Applications(NCA '03), Cambridge, Mass., April 2003. These applications also supporta topology with multiple routes between servers; however, thesubscriptions are only propagated along a single selected “best route”in a spanning tree. This limitation of propagating subscriptions along asingle selected route makes the system slow and recovery from a spanningtree link failure by dynamically switching to another route difficult.In addition, these publish/subscribe systems do not provide a mechanismto share the load among multiple available paths and do not supportreliable delivery.

In B. Segall, D. Arnold, J. Boot, M. Henderson, and T. Phelps, ContentBased Routing with Elvin4, AUUG2K, Can berra, Australia, June 2000, thepublish/subscribe system is architectured around a single server thatfilters and forwards messages directly to consumers. The system,however, does not address the issues of scalability or availability.

The publish/subscribe system discussed in A. Snoeren, K. Conley and D.Gifford, Mesh-Based Content Routing using XML, Proceedings of the 18thACM Symposium on Operating System Principles (SOSP 2001), Alberta,Canada, October 2001 attempts to improve reliability with low latency bysending messages simultaneously over redundant links in a mesh-basedoverlay network. The protocol uses content-based routing and provides ahigh level of availability. However, there is no guarantee of in-order,gapless delivery when subscriptions are dynamically added and removedfrom the system.

G. Cugola, E. Di Nitto, and A. Fuggetta, The JEDI Event-BasedInfrastructure and Its Application to the Development of the OPSS WFMS,IEEE Transactions on Software Engineering, 27(9):827-850, September 2001discusses a publish/subscribe system that guarantees causal ordering ofevents, as a special case, The ordering of events is published by anentity called the Active Object. This system provides twoimplementations of the event dispatcher. The first version is acentralized version constituting a single process and addressing therequirements of simple systems. The second version is a distributedversion constituted of a set of dispatching servers interconnected intoa tree structure. This distributed version, while addressing part of theneeds of Internet-wide distributed applications engaging in intensecommunication, does not accommodate and utilize redundant links betweendispatching servers and hence is neither highly available nor easilyused for load sharing.

The publish/subscribe system illustrated in B. Zhao, L. Huang, A.Joseph, and J. Kubiatowicz, Exploiting Routing Redundancy Using aWide-area Overlay, Technical Report UCB/CSD-02-1215, University ofCalifornia, Berkeley provides fault tolerant routing by dynamicallyswitching traffic onto pre-computed alternate routes. Messages in thissystem can be duplicated and multicast “around” network congestion andfailure hotspots with rapid re-convergence to drop duplicates. However,this system does not support content routing.

A. Rowstron, A. Kermarrec, M. Castro, and P. Druschel, SCRIBE: Thedesign of a Large-Scale Event Notification Infrastructure, Proceedingsof 3rd International Workshop on Networked Group Communication (NGC2001), UCL, London, UK, November 2001 describes a large-scale and fullydecentralized event notification system built on top of a peer-to-peerobject location and routing substrate overlaid on the Internet. Theevent notification system leverages the scalability, locality,fault-resilience and self-organization properties of the object locationand routing substrate. However, the event notification system does notsupport content-based routing. In addition, the event notificationsystem builds a separate multicast tree for each individual topic. Thismulticast tree is created using a scheme similar to reverse pathforwarding, a description of which can be found in Y. Dalal and R.Metcalfe, Reverse Path Forwarding of Broadcast Packets, Commnunicationsof the ACM, 21(12):1040-1048, 1978, so the route on which subscriptionmessages were forwarded are inverted to become the route by which eventsare later distributed. This makes it impossible to add a redundant nodeto the multicast tree to share the load without requiring the totalmulticast tree to be rebuilt. Although the system can recover frommulticast node failures by building a new multicast tree, this is doneat a cost of reliable, in-order, gapless delivery. The applications mustimplement higher quality of service by themselves. In addition, anun-subscription in the event notification system has to be delayed untilthe first event is received.

Therefore, a need exits for a publish/subscribe system that provides fora guaranteed in-order, gapless content-based routing of messages whilealso achieving high performance, scalability and high availability. Inaddition, the publish/subscribe system should not require consensus oragreement between the redundant routing members, enabling them to serveas routing and processing alternatives to each other for fault toleranceand load sharing.

SUMMARY OF THE INVENTION

The present invention is directed to a publish/subscribe system and amethod for propagating subscriptions and routing published messagesthrough the system. The publish/subscribe system contains publishersthat generate messages and subscribers that register interest in thosemessages. The system also includes a plurality of routing brokersoperatively coupled to and in communication with one another over aredundant overlay network. Each broker can be configured as a publisherconnecting broker, a subscriber connecting broker, an intermediatebroker or combinations thereof. Each publisher connecting broker is incommunication with one or more of the publishers, and each subscriberconnecting broker is in communication with one or more subscribers. Thesubscriber connecting brokers are operatively coupled to the publisherconnecting brokers via the network through the intermediate brokers.Therefore, messages are published to the system through the publicationbrokers, and subscriptions are propagated to the system through thesubscription brokers.

The overlay network can be viewed as a spanning tree of cells connectedby link bundles. The cells are nodes in the spanning tree topology andcan be, for example, routers or servers. Although each broker can be oneof the nodes in the spanning tree, preferably, the routing brokers arelogical brokers. Therefore, each node can have one or more brokersassociated with it, and each broker can have a presence in more than onenode in the tree. Similarly, each link bundle can include more than onelink. As part of the spanning tree topology, the nodes include rootnodes, intermediate nodes and leaf nodes. Publisher connecting brokersare located at the root nodes, and subscriber connecting brokers arelocated at the leaf nodes. Therefore, the location where the publisherconnecting brokers reside is referred to as upstream, and the locationwhere the subscriber connecting brokers reside is referred to asdownstream.

The system and method of the present invention are directed tocontent-based publish/subscribe systems in which each subscriber canrequest delivery of a unique set of messages. The system and method ofthe present invention provide subscription information for content-basedrouting in support of scalable, high performance, gapless messagedelivery. In addition, the publish/subscribe system of the presentinvention can operate asynchronously, i.e. the need for agreement orconsensus among the redundant routes and routing brokers is eliminated.

Each published message is associated with a set of message subscriptionsS_(m) that identify the subscriptions to which each published message isto be delivered, subject to content matching. The publisher connectingbrokers maintain a set of subscriptions and associate the appropriateset of messages subscriptions with messages published by publishersconnected to them. Associated with each broker is a set of brokersubscriptions S_(b) that identifies the list of subscriptions for whichthat broker maintains information such as routing information andinformation regarding the status of the subscription. As each message ispublished to the system, the message is routed through the overlaynetwork tree and through the brokers, from a publisher connecting brokerto a subscriber connecting broker and if necessary through one or moreintermediate brokers. The subscriber connecting broker is incommunication with the subscribers associated with the subscriptions andmaintains the most current information about each subscription. Eachsubscriber connecting broker uses this subscription information todeliver the published messages to the subscribers.

As the messages are published through the brokers, these messages aredirected in accordance with the current active subscriptions so that theproper messages can be routed to the subscriber connecting brokers thatneed those messages for delivery to the subscribers. Proper routingthrough the brokers is accomplished by comparing S_(b) to S_(m) at eachbroker for each published message routed through that broker in additionto the result of content matching. Based upon this comparison and inparticular on an identification of subscriptions that are bothassociated with the message and capable of being routed by the broker,the message is routed to downstream brokers and subscribers. Forexample, when the comparison indicates that all of the subscriptionscontained within S_(m) are also contained within S_(b), i.e. S_(b) isequivalent to or a superset of S_(m), then the broker containssufficient information regarding the routing of the message to itsdownstream brokers and subscribers and can match the subscriptions inS_(m) to the subscriptions in S_(b) and route accordingly. If S_(b) isless than S_(m), i.e. if S_(m) contains at least one subscription thatis not contained in S_(b), then the broker lacks all of the informationnecessary to filter and route the subscriptions in S_(m). Therefore, thebroker routes the messages to its downstream brokers regardless of thematching result.

Due to effects such as latency in the publish/subscribe system and theaddition and removal of subscriptions over time, a time element isassociated with each subscription in S_(m) and S_(b). The set of messagesubscriptions S_(m) is represented as a message vector V_(m) containinga plurality of message vector elements that are preferably integers.Each message vector element represents a single subscriber connectingbroker in the publish/subscribe system and contains the identificationof all active subscriptions in S_(m) associated with that subscriberconnecting broker and having an associated virtual start time that isless than or equal to the vector element. The set of brokersubscriptions S_(b) is represented as a broker vector V_(b) containing aplurality of broker vector subscription elements that are preferablyintegers. Each broker vector subscription element represents a singlesubscriber connecting broker in the publish/subscribe system andcontains the identification of all active subscriptions in S_(b)associated with that subscriber connecting broker and having anassociated virtual start time that is less than or equal to the vectorelement.

The comparison of S_(m) and S_(b) is accomplished by comparing V_(m) toV_(b) on an element-by-element basis, that is comparing the activesubscriptions in each message vector element to the active subscriptionsin each broker vector element. This comparison is referred to as thesufficiency test, i.e. whether the broker has sufficient knowledge ofall the subscriptions required by the published message for routing. Ifthe comparison indicates that all of the active subscriptions containedin the message vector elements are also contained in the broker vectorelements, then the broker associated with V_(b) can perform subscriptionmatching and routing in accordance with this matching. If the comparisonindicates that the message vector elements contain at least onesubscription not contained in the broker vector elements, then themessage is routed to all brokers downstream of the broker associatedwith V_(b). When comparing V_(m) to V_(b), the message vector and brokerelements are compared to confirm that the broker vector elements arelater than or equal to the corresponding message vector elements. Byconfirming that the broker vector elements are later than or equal tothe message broker elements, the system confirms that the subscriptionslocated at the broker are at least as recent as the subscriptionsassociated with the published message. Therefore, the system avoidsusing old subscription information in the routing of newer publishedmessages.

In general, virtual start times are assigned to new subscriptions bytheir connecting brokers. Each subscriber broker maintains a clock, forexample a monotonic, integer-valued counter. New subscriptions areintroduced to the publish/subscribe system of the present invention bysubscribers through associated subscription connecting brokers, andthese subscription brokers assign virtual start times to the newsubscriptions based upon the values of their clocks. Activesubscriptions, therefore, have virtual start times that are equal to orearlier than the current value of the clock contained in theirassociated brokers. Active subscriptions are those subscriptions thathave not unsubscribed as of the current time.

The message vector element for each subscriber connecting brokerindicates that the published message associated with the message vectorshould be delivered to all active subscriptions having a vector virtualstart time earlier than or equal to the corresponding message vectorelement for the relevant subscription connecting broker. Thus, the V_(m)vector associated with a published message forms a binding contractbetween the message and the brokers through which it is routed. Thebrokers match and filter the published message against all subscriptionshaving virtual start times earlier than the corresponding message vectorelements for the relevant connecting brokers. However, if a broker lacksinformation about a subscription associated with the published message,then that broker does not route the published message in accordance withmatching and filtering. Instead, the broker forwards the publishedmessage to all downstream brokers.

The method and system of the present invention consolidate or aggregatesubscriptions using covering relationships. In general, eachsubscription can be defined as a plurality of logical conjunctions. Eachconjunction in the plurality of conjunctions defines or specifies a setof published messages that are associated with the subscription. Theselogical conjunctions are aggregated so that repetitive conjunctions canbe removed, thus reducing the number of conjunctions that have to beanalyzed or considered by each broker during matching and routing of thepublished messages. Aggregation of the subscriptions can occurthroughout the publish/subscribe system. The logical conjunctions of thesubscriptions can be aggregated at subscriber connecting brokers, forexample, as new subscriptions are added to the system. Alternatively,the logical conjunctions associated with the subscriptions can beaggregated at all brokers as subscriptions are added and propagatedthrough the system.

Aggregation is facilitated by maintaining directed acyclic graphs(DAG's) at one or more of the brokers in the system. Each DAG representsthe logical conjunctions of each subscription in the S_(b) associatedwith the broker at which the DAG is maintained. The maintenance of theDAG includes adding new conjunctions to the DAG and removing expiredconjunctions from the DAG. Since each DAG is transitive along itsbranches, transitive arcs can be omitted from the DAG. When maintainingthe DAG, covering relationships are used to identify uncoveredconjunctions defining uncovered sets of published messages that are notthe subset of any other set of published messages defined by otherconjunctions. Therefore, uncovered conjunctions define unique sets ofpublished messages. In addition, these covering relationships identifycovered conjunctions that define covered sets of published messages thatare subsets of one of the uncovered sets of published messages.Therefore, a covered conjunction does not define or identify a publishedmessage that is not already identified by another conjunction.

Since each subscriber connecting broker maintains a clock and uses thisclock to assign virtual start times to subscriptions, the assigning ofvirtual start times is coordinated with the aggregation of thesubscriptions. Each uncovered conjunction that is added to the DAG isassigned a virtual start time that is equivalent to the current value ofthe clock associated with the broker when the uncovered conjunction isadded to the DAG. The clock time is advanced at the time the brokerpropagates the subscription changes. Covered conjunctions are assignedvirtual-start times that are the minimum values of the virtual starttimes associated with its covering conjunctions. Since each subscriptioncan contain a variety of covered and uncovered conjunctions, differentvirtual start times can be associated the different conjunctions of thesame subscription. Overall, the virtual start time associated with eachactive subscription is set equal to the maximum value, i.e. the latesttime, of the covered and uncovered conjunction virtual start times forthe conjunctions contained in the active subscription.

The DAG from a subscriber connecting broker, is propagated through oneor more additional brokers in the system. This propagation can takeplace in accordance with a pre-determined time interval or in responseto a change in the DAG. Propagation of the DAG includes propagating anychanges to the uncovered or root conjunctions in the DAG, identifyingthe broker and possibly the node or cell propagating the DAG,identifying the current time on the clock contained in the identifiedbroker at the time the change occurred, identifying the uncovered orroot conjunctions to be removed from the DAG, identifying the uncoveredor root conjunctions to be added to the DAG and providing a constraintvector. The constraint vector is a constraint on the value of the brokervector associated with the broker receiving the propagated change in theDAG. The form of the constraint vector is similar to the broker vectorin that it is a vector of virtual times, with one element for eachbroker that can be associated with a subscriber. For example, theconstraint vector identifies the oldest acceptable time, or virtualstart times, for subscriptions in the broker vector of the receivingbroker in order for the receiving broker to be able to accept andprocess the change. If the broker vector associated with the receivingbroker is too old or out-of-date, then it may be missing intermediatechanges that are necessary to process the current DAG changes.

Having received the propagated DAG changes and the associated constraintvector, the system confirms that the broker vector associated with thereceiving broker satisfies the constraint vector. If the constraintvector is satisfied, the changes to the DAG are applied and the brokervector is updated accordingly, such as for example advancing thecorresponding values in the broker vector. If the constraint vector isnot satisfied, then the DAG changes cannot be applied. The system,however, can request an update from the sending broker of the lateststate.

Subscriptions can be propagated as a best effort, hop-by-hoppropagation. If redundant brokers exist in a given hop, the subscriptionmessage can be propagated farther without an agreement being reachedamong the redundant brokers. Therefore, each broker can handle apublished message for a subscription even before it knows about thesubscription, which makes a failover extremely easy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of an embodiment of apublish/subscribe system in accordance with the present invention;

FIG. 2 is a schematic representation of an embodiment of a broker foruse in the publish/subscribe system;

FIG. 3 is a schematic representation of an embodiment of a directedacyclic graph for use at a broker;

FIG. 4 is a schematic representation of another embodiment of a directedacyclic graph for use at a broker;

FIG. 5 is a schematic representation of an embodiment of a directedacyclic graph at a broker before processing a subscription update;

FIG. 6 is a schematic representation of an embodiment of a directedacyclic graph at a broker after processing a subscription update FIG. 7is a flow chart illustrating an embodiment of a subscription propagationmethod in accordance with the present invention; and

FIG. 8 is a flow chart illustrating another embodiment of thesubscription propagation method.

DETAILED DESCRIPTION

An embodiment of a content-based publish/subscribe system in accordancewith the present invention includes one or more publishers posting orpublishing messages to one or more subscribers across a networkcontaining a plurality of brokers. Suitable networks include local areanetworks (LAN), wide area networks (WAN) such as the Internet or WorldWide Web, private area networks, secure area networks, public switchedtelephone networks (PSTN) and combinations thereof. The network ispreferably an overlay network containing a plurality of connected cellsor nodes. The brokers are disposed at one or more of these nodes and arein communication across the network. The plurality of brokers providefiltering and routing of published messages to the subscribers based, atleast in part, on the content of the published messages and on filteringpredicates provided by the subscribers. Therefore, redundant routesexist from each publisher to each subscriber through the plurality ofbrokers. The present invention allows a publish/subscribe system to takeadvantage of this redundancy in delivering published messages throughthe publish/subscribe system. The present invention, however, canfunction in the absence of this redundancy.

Methods in accordance with the present invention propagate subscriptionsthrough the publish/subscribe system so that not only best-effortdelivery but also in-order, gapless delivery, also referred to asreliable delivery, of published messages is supported. Thepublish/subscribe system can contain a plurality of published messagestreams. In reliable delivery, the publish/subscribe system finds astarting published message in each of the published message streams foreach subscription and from that published message in the publishedmessage stream delivers all and only those published messages matchingthe subscription in an order consistent with the original publishedmessage stream.

For simplicity, embodiments are presented for reliable delivery from asingle published message stream. However, each broker within thepublished/subscribe system can handle multiple, simultaneous publishedmessage streams and can perform different filtering and routing roleswith respect to each one of the published message streams. When a singlepublished message stream is considered, the brokers through which thesingle message stream is routed can be distinguished based upon thefunctionality these routers assume with respect to the published messagestream. These functionalities include, but are not limited to, acting asa publisher hosting or connecting broker (PB), acting as an intermediateor routing broker (IB) and acting as a subscriber hosting or connectingbroker (SB). The PB is the broker that is in first or immediate contactwith a publisher and hosts the published message stream generated bythat publisher. Similarly, the SB is the broker that is in first orimmediate contact with a subscriber and hosts the subscriber includingaccepting subscriptions from the subscriber and propagating thosesubscriptions to the publish/subscribe system. Any number of IB's canexist between each PB and each SB. These IB's perform routing andfiltering of published messages and propagating of subscriptions. In oneembodiment, each broker is an independent entity having a singlefunctionality. In another embodiment, any one broker performs two ormore functionalities. Preferably, each broker is a logical broker orconstruct associated with the nodes or cells in the publish/subscribesystem and representing either a single logical broker or a plurality oflogical brokers. These nodes can represent routers, servers, computersor other hardware in the overlay network within the publish/subscribesystem. In one embodiment, each broker is associated with a single node.In another embodiment, each broker participates in a plurality of nodes.For example, each broker can be a process or application running on acomputer, or, to minimize delay due to system failures, a cluster ofredundant processes nming in a distributed manner within multiplecomputers.

Referring initially to FIG. 1, an embodiment of a publish/subscribesystem 10 in accordance with the present invention is illustrated. Thepublish/subscribe system 10 contains a plurality of publishers 101 a,101 b, 101 c. Each publisher is capable of publishing messages orstreams of messages through the publish/subscribe system 10. The systemalso includes a plurality of subscribers 105 a, 105 b, 105 c, 105 d, 105e, that are users or clients in the publish/subscriber system 10. Eachsubscriber is capable of creating and introducing into the system 10 oneor more subscriptions that identify one or more sets of publishedmessages that the subscriber wants to receive. Each subscriber is alsocapable of receiving published messages. Disposed between thesubscribers and the publishers are a plurality of brokers 12. Asillustrated, each broker 12 is associated with one or more nodes, N₁₁,N₁₂, N₁₃, N₂₁, N₂₂, N₃₁, in the system 10 forming an overlay network ofredundant brokers between the subscribers and the publishers. The nodesrepresent routers, servers and other devices within the network.

Each broker 12 can serve one or more functions within the system 10. Asillustrated, these functions include PB 102, SB 106 a, 106 b, 106 c andIB 109 a, 109 b, 109 c, 109 d. Publishers 101 a, 101 b, 101 c connect tothe publish/subscribe system 10 through one or more PB's 102 acrosspublisher connections 103 a, 103 b, 103 c. In particular, the publisherconnections run between a publisher and a PB 102 disposed at a node N₃₁.Suitable publisher connections include any type of communication mediumfor conveying transmitted information, including a wirelesscommunication link, such as, for example, infrared, radio frequency,satellite and microwave, and a dedicated communication connection, forexample, telephone, cable and fiber optic. Preferably, the publisherconnections are a reliable, first-in-first-out (FIFO) connection,including but not limited to, a Transport Control Protocol/InternetProtocol (TCP/IP) socket connection.

Subscribers 105 a, 105 b, 105 c, 105 d, 105 e connect to thepublish/subscribe system 10 through one or more subscriber connections108 a, 108 b, 108 c, 108 d, 108 e. In particular, the subscriberconnections run between one or more subscribers 105 a, 105 b, 105 c, 105d, 105 e and one or more SB's 106 a, 106 b, 106 c. Suitable subscriberconnections are the same as for the publisher connections. The PB 102and SB's 106 a, 106 b, 106 c are connected to the plurality of IB's viainterbroker connections 111 a, 111 b, 111 c, 111 d, 111 e, 111 f, 111 g,111 h, 111 i, 111 j. In one embodiment, the interbroker connectionsutilize FIFO protocols, for example TCP/IP. Preferably, the interbrokerconnections utilize a faster, less reliable protocol since the deliveryprotocol of the present invention tolerates connection failures andmessage reordering, thereby increasing system throughput.

As illustrated, the arrangement of brokers 12 in the system 10constitutes a logical tree structure. Root nodes in the tree are locatedclose to the publishers and are associated with PB's. Leaf nodes arelocated close to the subscribers and are associated with SB's. IB'sconstitute the intermediate nodes in the tree structure. The interbrokerconnections form the branches of the tree structure. Messages introducedinto the system 10 are published through the IB's in the tree from theroot nodes to the leaf nodes. Subscriptions are propagated up throughthe IB's in the tree from the leaf nodes to the root nodes. Filteringand routing of the published messages is provided at each broker inaccordance with the propagated subscriptions to provide for accurate andreliable delivery of the desired messages to the SB's and subscribers.

In one embodiment, each published message is associated with a set ofmessage subscriptions S_(m) that identify the subscriptions to whicheach published message is to be delivered. Each PB maintains a set ofsubscriptions for its connected publishers and generates an appropriateS_(m) for each message published by these connected publishers.Associated with each IB and SB is a set of broker subscriptions S_(b)that identifies the list of subscriptions for which each brokermaintains information such as routing and delivery information andinformation regarding the status of the subscription. As each message ispublished through the system, the message is routed through the overlaynetwork tree, from a PB through to one or more SB's and possible throughone or more IB's. Each SB maintains the most current and completeinformation about each subscription for its connected subscribers. EachSB uses this subscription information to deliver the published messagesto the subscribers.

As the messages are published through the brokers, these messages aredirected in accordance with the current active subscriptions so that theproper messages can be routed to the subscriber connecting brokers thatneed those messages for delivery to the subscribers. Proper routingthrough the brokers is accomplished by comparing S_(b) to S_(m) at eachbroker for each published message routed through that broker. Based uponthis comparison and in particular on an identification of subscriptionsthat are both associated with the message and capable of being routed bythe broker, the message is routed to downstream brokers and subscribers.For example, when the comparison indicates that all of the activesubscriptions contained within S_(m) are also contained within Sb_(b),i.e. S_(b) is equivalent to or a superset of S_(m), then the brokercontains sufficient information regarding the routing of the message toits downstream brokers and subscribers and can match the publishedmessage using the subscriptions in S_(b) and route accordingly. If S_(b)is less than S_(m), i.e. if S_(m) contains at least one subscriptionthat is not contained in S_(b), then the broker lacks all of theinformation necessary to filter and route the subscriptions in S_(m).Therefore, the broker routes all of the messages in S_(m) to all of itsdownstream brokers.

In addition to identifying the types of messages desired and thesubscribers desiring the messages, the present invention associates atime element with each subscription. In one embodiment, a virtual starttime and a virtual end time is associated with each subscription. Thesevirtual start times and virtual end times convey temporal relationshipsand dependencies among the subscriptions and messages. Each SB maintainsa clock. Suitable clocks include 24 hour clocks, count-up timers,count-down timers, accumulators and counters including integer-valuedcounters. Preferably, the clock is a non-zero, integer-valued counterhaving an always increasing value. Clock values are preferablymonotonically assigned to subscriptions. Virtual start times areassigned to each subscription by one of the brokers. In one embodiment,each SB assigns a virtual start time to new subscriptions submitted bythe subscribers associated with the SB. The virtual start time isequivalent to the current value of the clock contained within the SB atthe time the new subscription is received.

The combination of subscriptions, for example the subscriptionscontained within S_(b) and S_(m), with virtual start times defines avector representation of the subscriptions associated with eachpublished message and the subscriptions maintained at each broker. Inone embodiment, the set of message subscriptions S_(m) is represented asa message vector V_(m) containing a plurality of message vectorelements. Each message vector element represents a single SB in thepublish/subscribe system and contains the identification of all activesubscriptions in S_(m) associated with that SB and having an associatedvirtual start time less than or equal to the value of the vectorelement. The set of broker subscriptions S_(b) is represented as abroker vector V_(b) containing a plurality of broker vector elements.Each broker vector element represents a single SB in thepublish/subscribe system and contains the identification of all activesubscriptions in S_(b) associated with that SB and having an associatedvirtual start time less than or equal to the value of the vectorelement. Preferably, a broker vector V_(b) is maintained at each broker.

By propagating the virtual start times with the subscriptions, therouting brokers maintain not only information about what kind of datamessages subscribing clients from each neighboring part of the networkare interested in, but also a short digest of what the broker knowsabout the subscriptions of clients from the neighboring network.

When a data message is published by one of the publishers through anassociated PB, a V_(m) is assigned to the published message. As themessage is routed or trasferred through the system, each broker comparesits V_(b) with the V_(m) associated with the message for the SB'slocated downstream. If for all downstream SB's, the V_(b) elements areat least the same as the V_(m) elements, the broker can use matching andfiltering to route the message. Therefore, if matching indicates thatthere are no matching subscribers downstream of the broker, the brokerfilters out the message and does not send the message downstream.However, if for some downstream SB's, V_(b) is less than V_(m), that isif V_(m) contains at least one element not contained in V_(b), thebroker sends the message to all downstream brokers regardless ofmatching. Therefore, a comparison of S_(b) to S_(m) is accomplished bycomparing V_(b) to V_(m).

V_(b) is established and maintained at each broker in accordance withsubscription information that is propagated up through the system fromthe subscribers to the publishers. In addition to propagating anidentification of the subscription itself and the virtual start timeassociated with the subscription, a constraint vector is also provided.By comparing this constraint vector to the V_(b) maintained by thebroker, the broker determines if it's V_(b) is equivalent to theconstraint vector on the elements for the subscriber connecting brokersshown in the constraint vector, i.e. if the subscription informationmaintained at the broker is up-to-date. If V_(b) 's elements are thesame as the constraint vector's for these subscriber connecting brokers,the subscription information maintained at the broker containssufficiently up-to-date information to accept and to apply any changesassociated with the propagated subscription. If, however, V_(b) does notsatisfy the constraint vector, the broker cannot apply the subscriptionchanges. Therefore, V_(b) needs to be updated before accepting the mostrecently propagated subscription.

As shown in FIG. 2, an embodiment of a node or cell 150 within thepublish/subscribe system that contains one or more brokers 12 isillustrated. The node 150 can be associated with any type of brokerincluding a PB, SB, IB and combinations thereof. As illustrated, thenode 150 contains a logical processor 152, a computer readable storagemedium 154 and one or more input/output (I/O) devices 156. Suitablelogical processors include, but are not limited to, a central processingunit (CPU) and a microprocessor. Although illustrated as a singlelogical processor, each node 150 can contain a plurality of logicalprocessors, and elements associated with a processing device can beshared by other processing devices. Suitable computer readable storagemediums include memory and other computer-readable media associated witha processor or CPU, including, but not limited to, random access memory(RAM), read only memory (ROM), fixed storage media such as hard diskdrives, removable storage media such as floppy diskettes, flash memoryand combinations thereof. Suitable I/O devices include, but are notlimited to keyboards, mice, network interface cards, printers, monitorsand combinations thereof.

Accordingly, an application program, or software components thereof,including computer executable instructions or code for performingmethods in accordance with the present invention is, in one embodiment,stored in one or more of the computer readable storage mediums 154. Whenthe computer executable code is executed by the processor 152, theprocessor 152 performs a method in accordance with the presentinvention. Alternatively, the computer executable code can be stored ina computer readable medium and executed by a processor that is externalto and independent of the nodes contained within the logical tree of thepublish/subscribe system 10. Suitable equipment for the node 150includes, but is not limited to a personal computer, a mainframecomputer and a network server.

As subscriptions are propagated through the brokers 12 in thepublish/subscribe system 10, these subscriptions are consolidated oraggregated. Any method capable of consolidating subscriptions whilemaintaining the necessary information for routing and delivery ofpublished messages can be used. For example, each subscription can berepresented as a disjunction of conjunctive filters by performing adisjunctive normal form (DNF) transformation of the Boolean expressionof the content filter. In one embodiment, the consolidation andaggregation method takes advantage of the conjunctive expressions of thesubscriptions. Preferably, the aggregation method utilizes coveringrelationships among the conjunctive filters. For example, when a brokerreceives a new propagated subscription containing a plurality ofconjunctions, the broker checks each new conjunction against all of theexisting conjunctions associated with subscriptions previously receivedby the broker and represented in the associated V_(b) to determine ifthe published message sets defined by any of the new conjunctions arecovered by any existing conjunctions. A new conjunction is covered by anexisting conjunction if all published messages matching the newconjunction also match the existing conjunction.

Referring to FIGS. 3-6, the conjunctions at each broker are presented asnodes in a structure with edges drawn from covering conjunctions tocovered conjunctions. Therefore, the subscriptions maintained at eachbroker, that is the subscriptions contained in S_(b) or V_(b), arerepresented by a partial order directed acyclic graph (DAG), which canalso be referred to as a conjunction DAG′. Conjunctions that are notcovered by any other conjunctions are called uncovered conjunctions andare represented as root nodes in the DAG. Therefore, logicalconjunctions within each DAG are aggregated so that covered conjunctionsdo not need to be propagated to upstream brokers, thus reducing thenumber of conjunctions that have to be analyzed or considered by eachbroker during matching and routing of the published messages. Since eachDAG is transitive along its branches, transitive arcs can be omittedfrom the DAG. In one embodiment, aggregation is performed at each SB,for example as new subscriptions are introduced into the system 10. Inanother embodiment, aggregation of subscriptions is performed throughoutthe publish/subscribe system 10 at each broker 12 as subscriptions areadded and propagated through the system.

Each DAG represents the logical conjunctions of each subscription in theS_(b) associated with the broker at which the DAG is maintained.Maintaining each DAG includes adding new conjunctions to the DAG andremoving expired conjunctions from the DAG. When maintaining each DAG,covering relationships are used to identify uncovered conjunctionsdefining uncovered sets of published messages that are not the subset ofany other set of published messages defined by other conjunctions.Therefore, uncovered conjunctions define unique sets of publishedmessages. In addition, these covering relationships identify coveredconjunctions that define covered sets of published messages that aresubsets of one of the uncovered sets of published messages. Therefore, acovered conjunction does not define or identify a published message thatis not already identified by another conjunction. The uncoveredconjunctions are maintained and propagated to brokers in the upstream ofthis broker, and covered conjunctions while maintained, for example, inthe SB's are not propagated upstream through the brokers in the system.

Referring to FIGS. 3 and 4, a DAG 200 for SB₁ 106 a (FIG. 1) isillustrated both before and after the addition of a new subscription tothe DAG. As shown in FIG. 3, before addition of a new subscription, theDAG contains an existing conjunction filter 201. As illustrated, theexisting conjunction filter is “Stock=nyse:ibm”. The existingconjunction filter 201 is associated with at least one subscriber 105 ahaving a content filter conjunction containing the existing conjunction201. A new subscription is introduced into the system 10 from a secondsubscriber 105 b that is also associated with SB₁. As illustrated, thenew subscription is “Stock=nyse:t&p>19 or Stock=nyse:ibm&p>90 orStock=nasdaq:msft”. This new subscription contains a first newsubscription conjunction 252, “Stock=nyse:ibm&p>90”, a second newsubscription conjunction 253, “Stock=nyse:t&p>19”, and a third newsubscription conjunction 254, “Stock=nasdaq:msft”. These newsubscription conjunctions 252, 253, 254 are added to the DAG 200 of SB₁,aggregating in accordance with covering relationships. The updated DAG200 is illustrated in FIG. 4.

The second and third new conjunctions 253, 254 define sets of publishedmessages that are not contained within the set of published messagesdefined by the existing conjunction 201. Therefore, the second and thirdnew conjunctions 253, 254 are added to the DAG 200 as root nodes. Thefirst new conjunction 252, however, is covered by the existingconjunction since every published message in the set defined by“Stock=nyse:ibm&p>90” is also contained in the set of published messagesdefined by “Stock=nyse:ibm”. Therefore, the first new subscription 252is added to the DAG 200 as a child leaf node of the existing conjunction201. Directed edge 255 between the existing conjunction 201 and thefirst new conjunction 252 represents this covering relationship.

In addition to analyzing each new subscription for coveringrelationships and integrating the new subscription into the DAG inaccordance with these relationships, each SB assigns a virtual starttime to the new subscription based upon the value of the clockmaintained at the SB. In one embodiment, virtual start times areassigned to each conjunction in the new subscription, and the maximumvalue of these times is taken as the virtual start time of the newsubscription. As shown in FIGS. 3 and 4, the existing conjunction 201associated with an existing subscription has been assigned a virtualstart time 256 equal to the clock value 1. The value of the clockassociated with SB₁ has been incremented to the value of 2. When the newsubscription is added at SB₁, the first new conjunction 252 is coveredby the existing conjunction 201 and is assigned a virtual start time 257equal to the existing conjunction virtual start time 256 of 1. Ingeneral for new covered conjunctions, the assigned virtual start time isthe minimum value of the virtual start times of all its immediateparents. The second and third new conjunctions are not covered and areassigned virtual start times 258, 259 equal to the current value of theclock at SB₁, which is 2. The new subscription is thus assigned avirtual start time equal to 2, and the clock associated with SB₁ isincremented to the value 3 when SB₁ propagates this subscription change.This virtual start time is used by the system to determine the startingpoint in the stream of published messages for message delivery inaccordance with the new subscription.

As illustrated, the clock maintained by SB₁ is an integer valuedcounter. This SB ensures the monotonicity of the highest value of thisclock, even in the event of recovery from a system crash. Suitablemethods for ensuring monotonicity include using a monotonic system clockand persisting an upper bound on the highest virtual clock time.Preferably, the clock values in accordance with the present invention donot overflow, which is reasonable for a clock value that is 64 bits orlonger.

Referring again to FIG. 1, the SB's 106 a, 106 b, 106 c propagatesubscriptions through the system 10 toward the PB's. This subscriptioninformation includes the addition of new subscriptions and the removalof expired subscriptions that have uncovered conjunctions in the DAG.This information is propagated in accordance with certain configurablecriteria. In one embodiment, the subscription information is propagatedin accordance with pre-determined time intervals. In another embodiment,a SB propagates changes periodically if at least one new subscriptionaddition or removal request has been received during a period.Preferably, a SB propagates subscription information every time arequest to add or remove a subscription is received and if theadditional or removal of the subscription changes the root nodes in itsDAG. In one embodiment, when the SB propagate changes to the DAG, the SBpropagates the change with its current clock value and increases thevalue of its clock. The propagated changes instruct IB's 109 a, 109 bregarding the operations to be performed in order for these IB's to beable to match published data messages to the current subscriptions.

In one embodiment, the incremental changes are computed by indicatingthe root conjunctions nodes to be removed from or added to the DAG.Therefore, if there are no root conjunction nodes to be added orremoved, there is no need to propagate any subscription information inresponse to the addition or removal of a subscription. As illustrated inFIGS. 3 and 4, the new subscription containing “Stock=nyse:t&p>19 orStock=nyse:ibm&p>90 or Stock=nasdaq:msft” causes a subscriptioninformation to be propagated from SB₁ since only “Stock=nyse:ibm&p>90”is covered and “stock=nyse:t&p>19” and “stock=nasdaq:msft” are not. Thepropagated subscription information contains information including anidentification of the originating SB, a value for the constraint vectorand instructions for which conjunctions to add to or remove from theDAG. The constraint vector in the subscription information, for thisembodiment, instructs the receiving broker to process this subscriptioninformation after it processes propagated subscription informationhaving a virtual start time less than or equal to 1. Therefore, theconstraint vector provides for the proper sequencing of subscriptionupdates and the preservation of dependencies between subsequent updates.If subsequent to the addition of the new subscription, SB₁ receives theadditional subscription “Stock=nasdaq:msft&p>24” from a subscriber, asubscription change will not be propagated because the only conjunctionassociated with this additional subscription is covered by the nowexisting conjunction “Stock=nasdaq:msft” in the DAG.

In one embodiment, dependencies between subscriptions are limited tosubscriptions originating from the same SB. Therefore, upstream brokers102, 109 a, 109 b process subscription information changes having avirtual start time T from the SB after processing all subscriptioninformation changes from the same SB having virtual start times earlierthan T. In the subscription “Stock=nyse:t&p>19 or Stock=nyse:ibm&p>90 orStock=nasdaq:msft”, since “Stock=nyse:ibm&p>90” is covered and thus notincluded in the subscription information change, a dependency ofsubscriptions with virtual start time equal to 2 is established onsubscriptions with virtual start times equal to 1. In one embodiment,this constraint is simplified by assuming a dependency for allsubscriptions from the same SB having a certain virtual start time T onall subscriptions from the same SB with virtual start times earlier thanT. As the subscriptions are propagated through the system, eachsubscription change can be further aggregated with the existingsubscription DAG's at each broker. This continuous aggregation cancreated new dependencies across SB's.

Referring to FIGS. 5 and 6, a DAG 300 for IB, 109 a is illustrated bothbefore and after the propagation of subscription information. Asillustrated, the propagated subscription information is propagated fromSB₁ 106 a to IB₁ 109 a and contains an identification of SB₁ with avirtual start time equal to 2, a constraint vector equal to 1, aninstruction to add the conjunction filter “Stock=nyse:t&p>19” and aninstruction to add conjunction filter “Stock=nasdaq:msft”. As shown inFIG. 5, the DAG 300 includes an existing root conjunction 301,“Stock=nyse:*”, and an existing covered conjunction 302,“Stock=nyse:ibm”. Directed edge 303 runs between the existing rootconjunction 301 and the existing covered conjunction 302 and representsthe existing covering relationship.

The propagated subscription information results in a new coveredconjunction 304, “Stock=nyse:t&p>19”. This conjunction 304 is covered bythe existing root conjunction 301, “Stock=nyse:*”, as indicated by thedirected edge 306. Since the new conjunction is covered by the existingroot conjunction 301 that was received at IB₁ from SB₂ 106 b with avirtual start time equal to 1, i.e. SB₂:1, IB₁ does not need topropagate this new covered conjunction 304 further through the system10. However, a dependency is created between SB₁ and SB₂, i.e. SB₁:2depends on SB₂:1. This dependency is represented by the constraintvector. The propagated subscription information also results in a newuncovered root conjunction 305, “Stock=nasdaq:msft”. Therefore, IB₁propagates to PB₁ the consolidated subscription information containingan identification of SB₁ with virtual start time equal to 2, theconstraint vector (SB₁:1, SB₂:1) and the instruction to add theconjunction “Stock=nasdaq:msft”. PB₁ will process this message after itreceives and processes all previously propagated subscriptioninformation from SB, having a virtual start time equal to 1, and allsubscription information from SB₂ having a virtual start time equal to1.

In one embodiment, subscription information is aggregated at each SB,reducing the volume of subscription information propagated through thesystem, because subscriptions that are completely covered by existingsubscriptions do not incur changes outside of the SB. New subscriptionsthat are partially covered by existing subscriptions cause a reductionin the amount of propagated information because only conjunctions thatare not covered need to be propagated, as incremental updates. Coveredsubscriptions are dependent on existing subscriptions at the SB.Therefore, incremental updates are processed by any receiving broker inthe order indicated by their virtual clock times. In another embodiment,subscriptions are aggregated at each broker throughout the system. Inthis embodiment, the incremental updates are aggregated at intermediatebrokers before propagating them upstream towards the PB's. Thisaggregation causes dependencies between subscriptions at different SB's.These dependencies are captured in a constraint vector that accompanieseach incremental update. In order for the receiving broker to correctlyprocess the subscription message, it is required to have a V_(b) withthe same value as the value of the constraint vector.

In one example, IB₁ 109 a (FIG. 1) is associated with V_(b)={SB₁:v₁,SB₂:v₂} and it partially aggregates an incremental update from SB₁ thathad a result version v₁+1, and the resulting update now depends on(SB₂:v₂). The new update will have a constraint vector (SB₂:v₂). Theupstream broker PB₁ 102 a applies this update if its V_(b) vector is{SB₁:v₁, SB₂:v₂}, and the resulting state has V_(b)={SB₁:v₁+1, SB₂:v₂}.Therefore, aggregation of subscription information only at SB's mayresult in more subscription message traffic than aggregating everywhere.Each broker also maintains larger subscription state in the former casebecause conjunctions from different SB's that are covered by others needto be maintained separately. Alternatively, aggregation only at SB'sresults in simpler computation having less restrictions on when anincremental update can be applied.

When subscriptions expire or are otherwise removed from the system 10,these modifications are also processed and propagated through the systemtoward the publishers. For example, if a root covering conjunction isremoved from a DAG, all conjunctions that are directly covered by thisroot conjunction either explicitly or implicitly are resubmitted. Forexample, if IB₁ receives a request to remove the existing rootconjunction 301, “Stock=nyse:*”, the existing covered conjunction 302,“Stock=nyse:ibm”, and new covered conjunction 304, “Stock=nyse:t&p>19”,are resubmitted. Therefore, if IB₁ generates the command to remove theconjunction “Stock=nyse:*” from the DAG, it also generates the commandsto add conjunctions “Stock=nyse:ibm” and “Stock=nyse:t&p>19”. If thesecommands result in new root conjunction changes in the DAG, then thesechanges are propagated farther through the system toward the publishers.

PB₁ cannot process this updated subscription information if there areadditional conjunctions that depend on the removed conjunction“Stock=nyse:*”, until PB₁ is provided with these additionalconjunctions. These additional dependent conjunctions can be routed frompeer brokers to IB₁, for example IB₂ 109 b. Moreover, PB₁ might not beaware of these additional dependencies, because IB₂ would only havepropagated the root conjunction. In one embodiment, PB₁ is checked forconsistency with the V_(m) associated with IB₁, the intermediate brokergenerating and propagating the subscription information. That is, thereceiving broker's V_(m) is checked against the V_(m) associated withthe propagating broker, in particular on the subscription elements forthe downstream SB's in common. In general, this dependency modelestablishes an approach both for subscription aggregation andconsolidation and for aggregation propagation by the brokers. Anadvantage of the method in accordance with the present invention is thatthere is no need to remember all the individual subscriptions and theirvirtual start times associated with each conjunction DAG node. Instead,the overall V_(b) vector of each broker is used for the implementation.

The method and system in accordance with the present invention enablesflexible propagation of subscription information, i.e. the addition andremoval of subscriptions. An example of this advantage is illustrated bypropagating subscription change information using only a single link inthe plurality of bundled links connecting one cell to another cell. Thesubscription information reaches one broker in the cell and is thenforwarded to all brokers operating within the same cell. This method isutilized to minimize inter-cell communication, since inter-cellcommunication is typically more expensive than intra-cell communication.However, the effectiveness of methods in accordance with the presentinvention is independent of this feature.

Each broker processes the changes indicated in the subscriptioninformation if the V_(b) associated with the broker satisfies aconstraint vector conveyed with the subscription information. If theconstraint is not satisfied and the V_(b) associated with the broker isnot sufficiently up-to-date to accept the changes conveyed in thesubscription information, then the broker can initiate an updatesequence to bring its V_(b) into compliance with the constraint vector.In one embodiment, each broker maintains a cache of the recentlypropagated subscription change information. When either propagatedsubscription messages are lost or in response to other system failures,the subscription information stored in the cache is used to reestablishthe V_(b) instead of having to initiate a full subscription stateupdate.

Referring again to FIG. 1, messages are published by one or more of theplurality of publishers 101 a, 101 b, 101 c through their associated PB102. Each published message is assigned a position in a stream ofpublishing end points, called pubends, and is preferably logged topersistent storage within the PB. Before the PB forwards the publishedmessage downstream toward one or more SB's, it assigns a V_(m) to themessage. Preferably, the PB assigns monotonically non-decreasing V_(m)'sto published messages in the stream when each published message isforwarded through the system for the first time. For a given first andsecond published messages, if the second message is located later in thedata stream than the first published message, the V_(m) assigned to thesecond published message is element-wise greater than or equal to theV_(m) assigned to the first published message. In one embodiment, the PBassigns monotonically non-decreasing V_(m)'s to each published messagebased upon the subscription information currently maintained at the PB.

The V_(m) associated with each published message contains the set ofsubscriptions that the published message should be matched against as itis routed through the brokers in the publish/subscribe system. A brokercan selectively route and filter a published message if it containscurrent subscription information for all of the subscriptions containedin the V_(m) associated with a published message. For example, IB's 109a, 109 b, 109 c, 109 d do not selectively route or filter publishedmessages for which they do not maintain all of the associatedsubscriptions, either directly or indirectly through covering filters.The sufficiency of the V_(b) associated with a broker is tested bycomparing that V_(b) to the V_(m) assigned to the published message. Iffor SB's downstream of the broker, the value of the broker's V_(b)elements is not less than those of the V_(m) assigned to the publishedmessage, the broker can filter or remove published messages if none ofthe subscriptions maintained by the broker matches the publishedmessage. Alternatively, if the broker's V_(b) elements are less thanthose of the V_(m) assigned to the published message, a conservativerouting approach is taken and the published message is routed to alldownstream brokers.

Systems and methods in accordance with the present invention are alsoused to establish a starting point in the published message stream forthe delivery of published messages in response to a propagatedsubscription; As the entry point of subscriptions, each SB maintains themost complete and up-to-date subscription information for each of itsattached subscribers. In one embodiment, a SB determines the deliverystarting point for a subscription by comparing the virtual start timeassociated with that subscription with the virtual start time of thecorresponding subscription element in the V_(m) associated with thepublished message. If the V_(m) element is equal to or greater than,i.e. if the time associated with the V_(m) element is concurrent with orlater than, the subscription's virtual start time, delivery inaccordance with the subscription is started. If the system only providesfor the monotonicity of V_(m) for messages that are sent for the firsttime but not for resent messages, then only first-time messages are usedto determine this delivery starting point. In one embodiment, thevirtual start time assigned to each subscription time is used as thestarting point of delivery. The SB associated with the subscriber startsdelivery of published messages when it receives the first data messagehaving an associated V_(m) with an element for that SB equal to orgreater than the virtual start time of the subscription. This method andsystem in accordance with the present invention where published messagesare assigned non-decreasing V_(m)'s and routing brokers do not filterpublished messages for which they do not maintain information for all ofthe subscriptions up to the times specified in the published messageV_(m) guarantees gapless, in-order delivery of published messages foreach subscription. Additional discussion of gapless delivery and durablesubscriptions in a content-based publish/subscribe system is found inU.S. patent application Ser. No. 10/177,474, which is incorporatedherein by reference in its entirety.

Referring to FIG. 7, an embodiment of a method for subscriptionpropagation through a publish/subscribe system in accordance with thepresent invention is illustrated. A subscriber, for example a firstsubscriber 105 a (FIG. 1), submits a new subscription 410 to thepublish/subscribe system 10, and this new subscription is received by aSB₁ 420 associated with the subscriber, for example SB, 106 a (FIG. 1).In one embodiment, each subscription is represented as a disjunction ofconjunction filters, for example by performing a DNF of the Booleanexpression content filter. Therefore, each subscription can be referredto as a set of conjunctions. The SB associated with the subscriber thatsubmits the new subscription maintains a clock, a DAG and a V_(b) 421.The SB aggregates the conjunctions with its existing DAG and assigns avirtual start time to the new subscription in accordance with thisaggregation 430. In order to aggregate the conjunctions contained in thenew subscription, an unprocessed or un-aggregated conjunction from thesubscription is selected 431. This conjunction is then analyzed todetermine if it is covered by any existing conjunctions in the DAG 432.If the conjunction is covered, the virtual start time associated withthis conjunction is set to the minimum value of all the virtual starttimes of its immediate parents in the DAG 433. The covered conjunctionis added to the DAG but not included for purposes of propagation. If theconjunction is not covered, the new conjunction is added to the DAG as anew root and is assigned a monotonically non-decreasing valued virtualstart time equal to the current value of the clock maintained by the SB434. The SB checks to see if the new subscription contains any moreunprocessed conjunctions 435. If unprocessed conjunctions still exist,then another conjunction is selected and the process is repeated. If nomore unprocessed conjunctions exist, then the SB assigns a virtual starttime to the entire subscription that is equal to the maximum value ofthe virtual start times associated with the conjunctions 436, incrementsits clock 437 and reports the virtual start time of the subscription tothe appropriate subscriber 440.

The SB then determines, based upon pre-defined criteria, whether or notto propagate the aggregated subscription information to other brokersupstream of the SB 450. Preferably, the SB propagates subscriptioninformation regarding changes to its aggregated DAG. If adding orremoving a subscription does not result in a change to the root nodes inthe DAG, then no information is propagated to the upstream brokers. If adetermination is made to propagate, then the SB propagates the necessarysubscription information, including an identification of theconjunctions to be added or removed and its virtual clock time to one ormore upstream brokers 460. Each upstream broker, i.e. IB's, maintains aDAG and a V_(b). In one embodiment, the new subscription information isaggregated into the existing DAG when the subscription information ispropagated from the SB to an IB. The subscription information ispropagated upstream until it is ultimately communicated to the PBassociated with published messages in the set of any conjunctions thatare to be added to or removed from a DAG 470. The PB also maintains aDAG and a V_(b) 471 and can aggregate new subscription information intoits existing DAG.

Referring to FIG. 8, an embodiment of publishing a messaging through apublish/subscribe system in accordance with the present invention isillustrated. A publisher submits a data message to the system 505, andthat message is received by its associated PB. The PB maintainssubscription information in a DAG and a V_(b) 511. The PB assigns aV_(m) to the new published message 520. In one embodiment, the PBmaintains previously assigned V_(m)'s in persistent storage to guaranteethat the V_(m) values assigned to new messages are in monotonicallynon-decreasing order. In another embodiment, the PB uses its DAG andV_(b) to assign V_(m). In this embodiment, the monotonicity of V_(m)values assigned to data messages is not guaranteed, but the SB willcheck for monotonicity and send negative acknowledgements to retrievethe data messages with a V_(m) that is sufficiently big. The publishedmessage is then routed through the brokers in the system 521. In oneembodiment, the PB uses the conjunctions in its DAG to generate a listof next hop routing destinations for the published message and routesthe message accordingly. At each broker, the PB checks if the V_(m) iselement-wise smaller than or equal to the broker's V_(b) 530. If bothconditions are met, the broker can match, filter and forward thepublished message in accordance with the subscriptions maintained at thebroker 540. If this condition is not satisfied, then the broker forwardsthe message to all of its downstream brokers 550. Although illustratedfor a single broker, the V_(b) and V_(m) comparison is executed bothsequentially and in parallel for each broker in the network tree throughwhich the published message is routed. Eventually, the published messageis routed to one or more SB's 560. The SB, maintains a conjunction DAGand a V_(b) 561. This V_(b) contains only one element for the SB itselfbut maintains the most up-to-date and detailed subscription informationfor its attached subscribers. The SB generates a list of subscriptionsmatching the published data message, and checks for each newsubscription, i.e., subscriptions for which the SB has not delivered anydata message, whether to start deliver data message for it. The SB doesso by comparing the data message's V_(m) element for the SB with thevirtual start time of the subscription 570. If the data message's V_(m)element is no less than the virtual start time of the new subscription,the SB decides this is the delivery starting point for the subscription,and the SB delivers the data message to the appropriate subscriber 580.For subscribers for which delivery has already started and whenmonotonic non-decreasing V_(m) are assigned, the SB delivers the messageto the subscriber because the comparison of virtual start times isalready satisfied. If non-monotonic V_(m) assignment is used, the SBperforms monotonicity checking on each data message to ensure a latermessage in the stream has a V_(m) element for this SB that is no lessthan the earlier messages in the same stream. If the published messagevirtual start time is not greater than or equal to the virtual starttime associated with the subscription, the SB issues a negativeacknowledgment to the PB and requests that the published message beresent with a new V_(m).

V_(m) can be assigned to a message either monotonically ornon-monotonically. Assigning V_(m) monotonically to published messagesrequires the PB to persist the highest V_(m) it has ever assigned, aswas discussed earlier. The benefit of this approach is that V_(m) onlyneeds to be assigned to published messages and each SB does not need tocheck subsequent V_(m)'s once delivery has started for a subscription.Non-monotonic V_(m) assignment does not require persistence of thehighest V_(m) at a PB. However, a subsequent published message can havea lower V_(m). (SB₁,2) than a previous message, (SB₁,3), that is locatedearlier in the published message stream. If the subsequent publishedmessage is converted to a data message representing silence before itarrives at SB₁, then the subsequent published message might not beaccepted, because the SB could have already started delivery forsubscriptions with virtual start times equal to 3. Therefore, datamessages representing silence also need to be assigned V_(m) in additionto published messages. In the monotonic embodiment, algorithm V_(m)'sare assigned to published messages only, since that serves as anindication of the lower bound of the silence ticks that follow the datamessage. Silence ticks that do not have a high enough V_(m) are rejectedand negative acknowledgements are sent for these published messages.Negative acknowledgement messages contain V_(m) indicating the minimumV_(m) of the silence that is acceptable. For example, the negativeacknowledgements in the above example will have V_(m)={SB₁,3}, andtherefore an IB with silence ticks having V_(m)={SB₁,2} cannot respondto those negative acknowledgements. Negative acknowledgementconsolidation at IB's also takes into account the V_(m).

In one embodiment, brokers other than PB's can change the V_(m) assignedto published messages. The motivation for changing V_(m) at IB's is thatwhen subscriptions are aggregated at IB's, it is possible that an updatecan be completely aggregated into existing subscriptions, resulting inan update that is effectively empty. Therefore, this effectively emptyupdate can either continue to be propagated until it reaches the PB's,or the propagation of this empty update can be stopped. Preferably, theeffectively empty update is propagated until it reaches the PB's. Theempty incremental update communicates the latest V_(m) and V_(b)information to the PB's. If the empty updates are stopped, the PB's donot see the empty updates, and the IB's record the fact that a messagewith V_(m)=vv₁ should be automatically changed to a message withV_(m)=vv₂, where vv₂>vv₁, since all the incremental subscription updatesthat change the subscription state from vv₁ to vv₂ are empty. It ispossible that another broker in the same cell might not have receivedall the updates up to vv₂ and hence will not make the same change toV_(m). This can result in non-monotonic V_(m).

While it is apparent that the illustrative embodiments of the inventiondisclosed herein fulfill the objectives of the present invention, it isappreciated that numerous modifications and other embodiments may bedevised by those skilled in the art. Additionally, feature(s) and/orelement(s) from any embodiment may be used singly or in combination withother embodiment(s). Therefore, it will be understood that the appendedclaims are intended to cover all such modifications and embodiments,which would come within the spirit and scope of the present invention.

1. A method for propagating subscriptions to published messages across anetwork, the method comprising: associating a message vector for apublished message, the message vector comprising a set of messagesubscriptions to the published message and an associated message vectorvirtual start time for each message subscription; associating a brokervector with a broker in the network, the broker vector comprising a setof broker subscriptions to the published message and an associatedbroker vector virtual start time for each broker subscription; comparingthe message subscriptions in the message vector to the brokersubscriptions in the broker vector; and routing the published messagethrough the network in accordance with the comparison.
 2. The method ofclaim 1, wherein the step of routing the published message comprisesrouting the published message in accordance with one or more of thebroker subscriptions when each message subscription matches one of thebroker subscriptions in the set of broker subscriptions.
 3. The methodof claim 1; wherein the step of routing the published message comprisesrouting the published to all downstream brokers and subscribers when thecomparison of the message subscriptions indicates that at least onemessage subscription contained in the set of message subscriptions isnot contained in the set of broker subscriptions.
 4. The method of claim1, wherein the step of comparing message and broker subscriptionscomprises confirming that the broker vector virtual start timeassociated each one of the broker vectors is later than or equal to themessage vector virtual start time associated with the messagesubscription matching that broker subscription.
 5. The method of claim1, further comprising confirming that the broker vector virtual starttimes are earlier than a current value of a clock contained in thebroker associated with the set of broker subscriptions.
 6. The method ofclaim 1, further comprising: defining each message subscription and eachbroker subscription as a plurality of logical conjunctions, each logicalconjunction defining a set of published messages; and aggregating thelogical conjunctions.
 7. The method of claim 6, wherein the step ofaggregating the logical conjunctions comprises aggregating the logicalconjunctions associated with the set of broker subscriptions at thebroker associated with the set of broker subscriptions.
 8. The method ofclaim 6, wherein the step of aggregating the logical conjunctionscomprises aggregating the logical conjunctions at all brokers in thenetwork.
 9. The method of claim 6, wherein the step of aggregating thelogical conjunctions comprises maintaining a directed acyclic graphrepresenting the logical conjunctions associated with the set of brokersubscriptions at the broker associated with the set of brokersubscriptions.
 10. The method of claim 9, wherein the step ofmaintaining the directed acyclic graph comprises: identifying uncoveredconjunctions in the logical conjunctions, wherein the set of publishedmessages associated with each uncovered conjunction is not a subset ofany other set of published messages defined by the logical conjunctions;and identifying covered conjunctions in the logical conjunction, whereinthe set of published messages associated with each covered conjunctionis a subset of one of the published messages defined by one of theuncovered conjunctions.
 11. The method of claim 9, further comprisingpropagating the directed acyclic graph through one or more other brokersin the network.
 12. The method of claim 11, wherein the step ofpropagating the directed acyclic graph comprises propagating inaccordance with a predetermined time interval.
 13. The method of claim11, wherein the step of propagating the directed acyclic graph comprisespropagating in response to a change in the directed acyclic graph. 14.The method of claim 10, further comprising: assigning an uncoveredconjunction virtual start time to each uncovered conjunction, theuncovered conjunction virtual start time equivalent to a current valueof a clock associated with the broker when the uncovered conjunction isadded to the directed acyclic graph; and assigning a covered conjunctionvirtual start time to each covered conjunction, the covered conjunctionvirtual start time equal to the uncovered conjunction virtual start timeassociated with its covering conjunction.
 15. The method of claim 14,further comprising setting the broker vector virtual start timeequivalent to a maximum value of the covered and uncovered conjunctionvirtual start times for the logical conjunctions contained in the brokervector.
 16. The method of claim 14, further comprising: propagating thedirected acyclic graph through one or more other brokers in the network;and advancing the clock associated with the broker after propagating thedirected acyclic graph.
 17. The method of claim 16, further comprising:assuring monotonicity of the clock; and maintaining the clock as aninteger counter.
 18. The method of claim 9, wherein the step ofmaintaining the directed acyclic graph further comprises: adding newsubscriptions to the set of broker subscriptions; and removing expiredsubscriptions to the set of broker subscriptions.
 19. The method ofclaim 9, further comprising omitting transitive arcs from the directedacyclic graph.
 20. The method of claim 13, wherein the step ofpropagating the directed acyclic graph further comprises: identifying ahighest vector broker virtual start time that can be associated with anyof the broker subscriptions in order to process the propagated change inthe directed acyclic graph; and propagating the highest vector virtualstart time in a constraint vector.
 21. The method of claim 20, furthercomprising: confirming that each broker subscription in the brokervector associated with the broker receiving the propagated change in thedirected acyclic graph satisfies the constraint vector; updating thebroker vector in accordance with the propagated change if the constraintvector is satisfied; and updating the broker vector in accordance withthe constraint vector if the constraint vector is not satisfied.
 22. Acomputer readable medium containing a computer executable code that whenread by a computer causes the computer to perform a method forpropagating subscriptions to published messages across a network, themethod comprising: associating a message vector for a published message,the message vector comprising a set of message subscriptions to thepublished message and an associated message vector virtual start timefor each message subscription; associating a broker vector with a brokerin the network, the broker vector comprising a set of brokersubscriptions to the published message and an associated broker vectorvirtual start time for each broker subscription; comparing the messagesubscriptions in the message vector to the broker subscriptions in thebroker vector; and routing the published message through the network inaccordance with the comparison.
 23. The computer readable medium ofclaim 22, wherein the step of comparing message and broker subscriptionscomprises confirming that the broker vector virtual start timeassociated each one of the broker vectors is later than or equal to themessage vector virtual start time associated with the messagesubscription matching that broker subscription.
 24. The computerreadable medium of claim 22, further comprising: defining each messagesubscription and each broker subscription as a plurality of logicalconjunctions, each logical conjunction defining a set of publishedmessages; and aggregating the logical conjunctions.
 25. The computerreadable medium of claim 24, wherein the step of aggregating the logicalconjunctions comprises maintaining a directed acyclic graph representingthe logical conjunctions associated with the set of broker subscriptionsat the broker associated with the set of broker subscriptions.
 26. Thecomputer readable medium of claim 25, wherein the step of maintainingthe directed acyclic graph comprises: identifying uncovered conjunctionsin the logical conjunctions, wherein the set of published messagesassociated with each uncovered conjunction is not a subset of any otherset of published messages defined by the logical conjunctions; andidentifying covered conjunctions in the logical conjunction, wherein theset of published messages associated with each covered conjunction is asubset of one of the published messages defined by one of the uncoveredconjunctions.
 27. The computer readable medium of claim 25, furthercomprising propagating the directed acyclic graph through one or moreother brokers in the network in response to a change in the directedacyclic graph.
 28. The method of claim 26, further comprising: assigningan uncovered conjunction virtual start time to each uncoveredconjunction, the uncovered conjunction virtual start time equivalent toa current value of a clock associated with the broker when the uncoveredconjunction is added to the directed acyclic graph; and assigning acovered conjunction virtual start time to each covered conjunction, thecovered conjunction virtual start time equal to the uncoveredconjunction virtual start time associated with its covering conjunction.29. The computer readable medium of claim 28, further comprising settingthe broker vector virtual start time equivalent to a maximum value ofthe covered and uncovered conjunction virtual start times for thelogical conjunctions contained in the broker vector.
 30. The computerreadable medium of claim 27, wherein the step of propagating thedirected acyclic graph further comprises: identifying a highest vectorbroker virtual start time that can be associated with any of the brokersubscriptions in order to process the propagated change in the directedacyclic graph; and propagating the highest vector virtual start time ina constraint vector.
 31. The computer readable medium of claim 30,further comprising: confirming that each broker subscription in thebroker vector associated with the broker receiving the propagated changein the directed acyclic graph satisfies the constraint vector; updatingthe broker vector in accordance with the propagated change if theconstraint vector is satisfied; and updating the broker vector inaccordance with the constraint vector if the constraint vector is notsatisfied.